72 PLEISTOCENE OF INDIANA AND MICHIGAN. 



known, and several other outcrops of conglomerate are discussed in the reports of the Indiana 

 Survey. The writer's examination of the conglomerate at Split Rock led him to the conclusion 

 that it was formed from a very stony till rather than from an ordinary gravel and cobble deposit. 

 The matrix is largely calcareous clayey material instead of sand and this is what binds the 

 stones together so firmly. The stones include bowlders measuring 2 feet or more. 



PRE- WISCONSIN DEPOSITS BENEATH WISCONSIN DRIFT. 



The pre- Wisconsin drift found some distance inside the limits of the Wisconsin is best dis- 

 tinguished from the Wisconsin if it consists of a clayey till. Generally this till is much more 

 indurated than that of thte overlying Wisconsin drift and is thus recognized by the drillers. In 

 places, however, the induration is less general and may even be localized in certain layers 

 between which are layers of loose material; in such places it is not so easy to determine when 

 the drill enters the gravel or sand of the pre-Wisconsin. Probably a considerable number of the 

 heavy deposits of drift in central and northern Indiana are of pre-Wisconsin age, but as they 

 are largely sand and gravel or loose-textured material they can not easily be discriminated 

 from the Wisconsin. Buried soils found in several of the central counties and still farther 

 north in parts of Lagrange County, Ind., and Hillsdale County, Mich., at depths of 25 to 100 

 feet, probably mark the line of separation. 1 



In the vicinity of Ann Arbor, Mich., a considerable number of wells, situated along a promi- 

 nent moraine that traverses the Ann Arbor quadrangle in a northeast-southwest course, enter, at 

 a general depth of about 100 feet, though in places much less, an indurated till, which is thought 

 to be pre-Wisconsin. A similar till is found along the Huron River valley in the vicinity of 

 Ypsilanti, Mich., and is reported in many wells in that vicinity. Indurated till is present also 

 in the plain east of the Ann Arbor quadrangle and on the eastern slope of the "thumb" of 

 Michigan as far north as Sanilac County, several exposures having been found along streams 

 and along the shore of Lake Huron. (See Pis. XII and XIII.) The appearance of the till in this 

 district is strikingly similar to that of the Illinoian drift outside the Wisconsin drift in Illinois 

 and Indiana. Though generally blue-gray it is traversed by seams and vertical joints or cracks 

 filled with a brownish or deeply oxidized material as in the typical Illinoian outside the Wis- 

 consin. Along the Lake Huron shore it is so indurated that small streams cascade over it as 

 they might over a rock ledge, and at Ypsilanti it has the appearance of a reef in the river bed 

 in which potholes and other features commonly found in rock formations are present. Such 

 induration is seldom if ever found in the clayey parts of the Wisconsin drift. This fact and the 

 local occurrence of soil between the indurated drift and the overlying softer material make it 

 reasonably certain that the deposits are pre-Wisconsin. The presence of such drift in this 

 region is to be expected from the fact that the ice sheet which formed the Illinoian drift must 

 have passed across the southern peninsula of Michigan to reach Indiana and Illinois. For 

 details of the pre-Wisconsin deposits of central and northern Indiana and southern Michigan 

 see pages 63-67 289-290. 



STRIDE OUTSIDE OF THE WISCONSIN DRIFT. 



Observations of strise in Indiana outside the limits of the. Wisconsin drift are confined to 

 seven counties, Owen, Putnam, Parke, Clay, Vigo, Sullivan, and Greene, all in the southwestern 

 part of the State. The striae bear from southward to eastward and, though somewhat divergent 

 in the neighboring exposures, they all seem to fall in naturally with the movements of the 

 Illinois glacial lobe, on the southern border of whose territory they appear. The following list 

 comprises all which have come under the writer's notice : 



i For an account of buried soils at Lagrange and Valentine, Ind., see Water-Supply Paper U. S. Geol. Survey No. 21, 1S99, p. 26. 



